This application is a 371 application of PCT/J00/000745 filed on Feb. 10, 2000.
The present invention relates to processes and apparatus for treating a gas containing reducing substances such as malodorants or pollutants by performing hydrothermal reaction and electrolytic reaction. Combination of hydrothermal reaction and electrolysis is herein referred to as hydrothermal electrolysis. Gases that are treated by processes of the present invention (influent gases) include gaseous materials containing reducing substances such as low-molecular weight organics, volatile organic halogen compounds, ammonia, hydrogen sulfide and cyanide gases with the balance being nitrogen, argon, air and oxygen. The present invention relates to processes and apparatus for converting reducing substances contained in said influent gases into harmless components such as carbon dioxide gas, nitrogen gas, sulfide ion, chloride ion or the like.
Gases containing reducing substances have been conventionally treated by gas phase catalytic oxidation, activated carbon adsorption, UV oxidation, etc. Gas phase catalytic oxidation involves mixing a gas containing reducing substances with an oxidizer such as air and bringing said mixture into contact with a catalyst under conditions of almost normal pressure and 160-300xc2x0 C. to oxidatively degrade malodorous components into carbon dioxide gas, nitrogen and water, whereby various malodorous components such as ammonia, low-molecular weight amines, mercaptans and aldehydes can be treated. However, this gas phase catalytic oxidation often suffers from a local temperature rise in the catalytic layer caused by autogenous combustion when the gas contains a lot of reducing substances. If the untreated gas contains a nitrogen compound (ammonia, amines, etc.), such a local temperature rise produces a high NOx fuel gas. If the gas contains reducing substances containing sulfur in the molecule, SOx may be produced. If the influent gas contains an organic halogen compound, a halic acid is produced in the catalytic layer. Once a halic acid is produced, the catalyst is readily poisoned. The catalyst is also often poisoned when the influent gas contains a dust component such as phosphorous compounds, sulfur compounds, silica. Especially, noble metal catalysts are more liable to be poisoned. Even when reducing substances free from these heteroatoms such as low-molecular weight hydrocarbons are treated by gas phase catalytic oxidation, incomplete combustion products such as CO gas may be produced under some process conditions. In order to advance complete oxidative degradation in catalytic oxidation, the.temperature of the catalytic layer must be strictly controlled. If the temperature is too high, the catalyst may be deteriorated or NOx may be produced. If the temperature is too low, however, reducing substances remain undegraded in the effluent gas. Therefore, it is very difficult to treat some kinds or levels of reducing substances contained in the influent gas by catalytic oxidation.
Activated carbon adsorption involves retaining reducing substances in pores of activated carbon by physical or chemical adsorption. Activated carbon adsorption typically takes place at normal temperature and pressure. Needless to say, reducing substances adsorbed to activated carbon must be further treated by burning or other means. When the molecular size of reducing substances is not suited to the pore diameter of activated carbon or reducing substances have no functional group suitable to be adsorbed, any adsorptive effect may not be produced for smoothly advancing the treatment of the gas. When the gas contains water or dust, adsorptive ability may be greatly lowered. When the influent gas contains reducing substances at high concentrations, a large amount of activated carbon is needed so that the cost for regenerating or treating such a large amount of activated carbon rises.
UV oxidation involves mixing a gas containing reducing substances with air and irradiating said mixture with UV rays to make the mineralization of the reducing substances into carbon dioxide gas or the like by accelerating a radical chain reaction. When a bond that absorbs a specific UV wavelength exists in reducing substance molecules, UV oxidation allows efficient degradation because radical reactions are readily chained. Especially when the gas contains such a component as trichloroethylene, UV oxidation is sometimes effective. However, reducing substances must be contained at relatively high concentrations in the influent gas to improve the chain reaction because the reaction proceeds by a radical chain reaction in this UV oxidation. When organochlorine compounds are to be treated, a special care is required about by-products such as dioxin because main degradative reaction is a radical reaction. Even if target reducing substances are effectively degraded, they are not always degraded into harmless inorganic components such as carbon dioxide gas but high concentrations of carbon monoxide or the like may be produced as a by-product.
In this way, processes of the prior art for treating a gas containing various reducing substances still have problems.
As a result of careful studies to overcome the above problems to find a process capable of efficiently degrading a gas containing various reducing substances, we accomplished the present invention on the basis of the finding that various reducing substances can be efficiently degraded by treating the gas by the hydrothermal electrolytic process.
We previously filed a patent application based on the finding that an aqueous medium containing reducing substances can be treated to efficiently degrade the reducing substances while inhibiting generation of hydrogen gas and oxygen gas by performing hydrothermal reaction and electrolytic reaction at the same time under predetermined conditions (hydrothermal electrolytic reaction) (International Patent Application PCT/JP 98/03544; International Publication WO99/07641). The disclosure of International Application PCT/JP 98/03544 is incorporated herein as a whole as reference. As a result of later studies, we accomplished the present invention on the basis of the finding that this hydrothermal electrolytic reaction process can be applied to the treatment of a gas containing reducing substances to degrade and remove the reducing substances in the gas.
Accordingly, an aspect of the present invention provides a process for treating a gas containing reducing substances by hydrothermal electrolysis, comprising supplying a gas containing reducing substances into a reactor charged with an aqueous medium containing a halide ion under application of a direct current at a temperature of 100xc2x0 C. or more but not more than the critical temperature of said aqueous medium and at a pressure that allows said aqueous medium to be kept in the liquid phase.
The present invention also provides an apparatus for performing the process described above, ie, an apparatus for treating a gas containing reducing substances by hydrothermal electrolysis, comprising a reactor having an inlet for introducing the gas containing reducing substances and an outlet for discharging the effluent gas and capable of resisting the pressure of hydrothermal reaction, and a pair of electrodes for performing electrolysis in said reactor.
In the present invention, an aqueous medium containing a halide ion such as chloride ion is mixed with a gas containing reducing substances and the mixture is electrolytically reacted at predetermined high temperature and high pressure to oxidatively degrade the reducing substances. In electrolysis, oxidation reaction proceeds at the anode to produce oxygen gas or a halogen-based oxidizer such as a hypohalous acid. Generally, oxidation reaction readily proceeds in the presence of an oxidizer such as oxygen gas at high temperature and high pressure of hydrothermal reaction. In the present invention, reducing substances such as organics and ammonia can be effectively oxidatively degraded by performing hydrothermal reaction and electrolysis at the same time.
Electrode reactions that can proceed in hydrothermal electrolysis of the present invention are described below. At the anode, reactions (1), (2), (3) below seem to proceed.
2Xxe2x88x92xe2x86x92X2+2exe2x88x92xe2x80x83xe2x80x83(1)
where X represents a chlorine atom, bromine atom or iodine atom or any combination thereof.
H2Oxe2x86x922H+{fraction (1/20)}2↑+2exe2x88x92xe2x80x83xe2x80x83(2)
Organic+H2Oxe2x86x92CO2↑+H++exe2x88x92xe2x80x83xe2x80x83(3)
In formula (1), a halide ion is oxidized to produce a halogen molecule. When X is a chlorine atom, for example, chlorine gas is produced. In formula (2), water is electrolyzed to produce oxygen gas. In formula (3), an organic is directly oxidized at the anode. The reaction of formula (1) and the reaction of formula (2) compete with each other and which reaction prevails depends on the type of the anode, the halide ion concentration in the aqueous medium and other factors. For example, the reaction of formula (1) prevails when a chlorine-generating electrode is used at a specific halide ion concentration or more.
The halogen molecule produced at the interface between the anode and the electrolyte by formula (1) reacts with its neighboring water to produce a hypohalous acid and a hydrogen halide.
X2+H2Oxe2x86x92HX+HXOxe2x80x83xe2x80x83(4)
where X has the meaning as defined above.
Hypohalous acids are excellent oxidizers capable of oxidatively degrading reducing substances contained in aqueous media. When the reducing substance is an organic, for example, the organic seems to be oxidized by the reaction below.
Organic+HXOxe2x86x92CO2↑+H2O+HXxe2x80x83xe2x80x83(5)
where X has the meaning as defined above.
When the reducing substance is ammonia, ammonia seems to be oxidized by the reaction below.
2NH3+3HXOxe2x86x92N2↑+3HX+3H2Oxe2x80x83xe2x80x83(6)
Hypohalous acids are excellent oxidizers especially in acidic solutions and hydrogen ion is produced by formulae (2), (3), (4) or the like to tend to form an acidic environment near the anode at which a hypohalous acid is produced. Thus, the hypohalous acid seems to especially favorably act as an oxidizer near the anode.
When X is a chlorine atom, the oxidation reaction by the hypohalous acid seems to especially participate in the degradation of reducing substances.
When X is a bromine atom or an iodine atom, the halate ion may participate in the degradation of reducing substances. Hypohalite ions disproportionate in basic solutions to produce a halate ion and a halide ion.
3XOxe2x88x92xe2x86x922Xxe2x88x92+XO3xe2x88x92xe2x80x83xe2x80x83(7)
For example, the reaction of formula (7) may occur when the hypohalous acid moves toward the cathode by diffusion or the like. This is because hydroxide ion is produced by anodic reaction to tend to form a basic environment near the cathode. The rate of the disproportionation reaction of formula (7) is higher in the order of chlorine, bromine and iodine, and a halate ion can be quantitatively obtained with bromine and iodine (F. A. Cotton, G. Wilkinson, P. L. Gaus, xe2x80x9cBasic Inorganic Chemistryxe2x80x9d, Baifukan, 1991, 2nd ed., p. 379). Halic acids are strong acids and potent oxidizers.
In formula (2), oxygen gas is produced by the electrolysis of water. Here, an oxygen atom seems to be first produced at the interface between the anode and the electrolyte. Said oxygen atom has a higher activity as an oxidizer than molecular oxygen so that it can efficiently oxidize reducing substances. Even if molecular oxygen is produced, reducing substances can be oxidized by hydrothermal oxidation reaction.
When the reducing substance is an organic, oxidation reaction by oxygen proceeds by the formula below.
Organic+O2xe2x86x92CO2↑+H2Oxe2x80x83xe2x80x83(8)
As shown by formula (3), reducing substances such as organics or ammonia may be directly oxidized at the anode by electrode reaction. When the reducing substance is ammonia, for example, the reaction of the formula below may proceed.
2NH3xe2x86x92N2↑+6H++6exe2x88x92xe2x80x83xe2x80x83(9)
Thus, hydrothermal electrolysis according to the present invention includes many reaction mechanism by which reducing substances are efficiently oxidatively degraded at or near the anode.
On the other hand, possible reactions at the cathode are as follows.
Water is electrolyzed to produce hydrogen at the cathode.
2H2O+2exe2x88x92xe2x86x92H2↑+2OHxe2x88x92xe2x80x83xe2x80x83(10)
Here, the so-called cathodic protection against corrosion can be provided by using the reactor body as cathode.
A reaction may also proceed in which an oxidizer is reduced at the cathode. The oxidizer here includes an oxidizer produced at the anode such as a hypohalous acid and optionally an externally added oxidizer. Examples of reaction are shown by formulae (11), (12) and (13) below.
The hypohalous acid is reduced at the cathode.
xe2x80x83HXO+exe2x88x92xe2x86x92Xxe2x88x92+OHxe2x88x92xe2x80x83xe2x80x83(11)
Oxygen dissolved in the aqueous medium represented by O2(aq) in the formulae below is also reduced.
xc2xdO2(aq)+H2O+2exe2x88x92xe2x86x922OHxe2x88x92xe2x80x83xe2x80x83(12)
The following reaction would possibly occur as another reduction reaction of dissolved oxygen at cathode.
O2(aq)+2H2O+2exe2x88x92xe2x86x92H2O2(active oxygen)+2OHxe2x88x92xe2x80x83xe2x80x83(13)
At the cathode, the reactions of formulae (11), (12) and (13) in which an oxidizer is reduced compete with the reaction of formula (10) in which hydrogen is generated.
Our experiments revealed that the reactions of formulae (11), (12), (13) or the like in which an oxidizer is reduced proceed preferentially to the reaction in which hydrogen is generated in hydrothermal electrolysis. Correspondingly, hydrogen generation is inhibited in hydrothermal electrolysis to reduce the possibility of coexistence of oxygen gas and hydrogen gas in the reactor and thus to reduce the danger of explosion. The oxidizer such as a hypohalous acid is degraded at the cathode to eliminate the secondary treatment for detoxifying the oxidizer in the effluent. For example, a hypohalite ion is generated at a high concentration during electrolysis at room temperature. However, the generation of a hypohalite ion was scarcely detected during electrolysis at high temperature.
Irrespective of the reaction mechanism, reducing substances such as organics and ammonia can be oxidatively degraded while inhibiting generation of hydrogen gas or oxygen gas according to the present invention.
Reducing substances that can be degraded/treated by the present invention include low-molecular weight organics such as methane, ethane, propane, butane, methyl mercaptan, acetaldehyde; volatile organic halogen compounds such as trichloroethane, trichloroethylene, chloroform, various chlorofluorocarbons; ammonia; hydrogen sulfide; cyanide gases; etc.
In the present invention, reducing substances in the influent gas are basically thought to be dissolved in an aqueous medium and then electrolyzed in the aqueous medium.
In the present invention, hydrothermal reaction takes place at a temperature of 100xc2x0 C. or more but not more than the critical temperature of the aqueous medium and at a pressure that allows said aqueous medium to be kept in the liquid phase. Temperatures lower than 100xc2x0 C. are not preferred because the rate of hydrothermal reaction is lowered to extend the reaction time. However, the finding of the present invention cannot be directly applied for temperatures higher than the critical temperature because physical properties of aqueous medium significantly change. At the supercritical state, for example, the solubility of the electrolyte such as a halide ion greatly decreases and electric conductivity is decreased.
In the present invention, the hydrothermal electrolytic reaction temperature is preferably 120-370xc2x0 C., more preferably 140-370xc2x0 C.
In the present invention, said aqueous medium preferably contains a halide ion. The halide ion serves as a catalyst in the hydrothermal electrolytic reaction so that degradative reaction proceeds more effectively. When the influent gas contains an organic halide, the same catalytic effect is obtained.
Such halide ions include chloride ion (Clxe2x88x92), bromide ion (Brxe2x88x92), iodide ion (Ixe2x88x92) or any combination thereof, among which chloride ion or bromide ion is especially preferred. A halide ion-producing salt may be dissolved in the aqueous medium. An acid such as hydrogen chloride (HCl), hydrogen bromide (HBr) or hydrogen iodide (HI) may be contained in the aqueous medium.
The halide ion-producing salt may be an inorganic or organic salt. For example, a salt of an acid such as hydrogen chloride (HCl), hydrogen bromide (HBr) or hydrogen iodide (HI) with a base is preferably used. Inorganic salts include, for example, alkali metal halides such as sodium chloride, potassium chloride; alkali earth metal halide such as calcium chloride; ammonium halides such as ammonium chloride; and complex salts such as tris(ethylenediamine) cobalt (III) chloride, tris(2,2xe2x80x2-bipyridine) iron (II) bromide. Organic salts include tetraalkylammonium halides such as tetraethylammonium chloride. Addition salts of amines and hydrogen halides (eg, aniline/hydrogen chloride) are also suitable. Exhaust gas from underground water treatment plants often contains organic halides such as trichloroethylene.
The aqueous medium preferably contains 0.05 mmol/l or more of a halide ion, more preferably 0.5 mmol/l or more of a halide ion, most preferably 5 mmol/l or more of a halide ion. This is because the halide ion produces a hypohalous acid via the electrolysis of the aqueous medium to oxidize reducing substances in the aqueous medium.
The aqueous medium preferably contains 0.05 mmol/l or more of chloride ion (Clxe2x88x92), more preferably 0.5 mmol/l or more of chloride ion, most preferably 5 mmol/l or more of chloride ion.
More preferably, the reaction system contains an oxidizer so that active oxygen or the like are also generated at the cathode to further accelerate degradative reaction. This oxidizer source may be oxygen contained in the influent gas, and if it is insufficient, an external oxidizer may be added to inhibit hydrogen generation and thus avoid formation of an explosive mixed gas. Oxidizers that can be externally added for this purpose are preferably oxygen gas, ozone gas, hydrogen peroxide and hypohalous acids, more preferably oxygen gas. Oxygen gas may be a gas containing oxygen gas, eg, air is preferably used.
When an oxidizer is externally added in the present invention, the dose of the oxidizer is preferably in the range of 0.01-100 equivalents of the amount necessary to completely oxidize reducing substances contained in the influent gas. If the oxidizer dose is less than 0.01 equivalents, the electric costs rise because the oxidizer must be wholly electrically generated. If the oxidizer dose is more than 100 equivalents, however, energy is wastefully consumed to pressurize an excessive oxidizer.
When the gas to be treated is acidic, reducing substances can be dissolved in an aqueous medium in the reactor by adding an alkali to the aqueous medium so that the reducing substances can be effectively degraded by hydrothermal electrolysis. When the gas to be treated is basic, however, reducing substances can be dissolved in an aqueous medium in the reactor by adding an acid to the aqueous medium so that the reducing substances can be effectively degraded by hydrothermal electrolysis. The expression xe2x80x9cacidicxe2x80x9d or xe2x80x9cbasicxe2x80x9d as referred to a gas herein is used to designate a gas showing a pH lower or higher than 7, respectively, when dissolved in water.
Even if reducing substances are not initially contained in a gas, they can be treated by this process by concentrating and transferring the reducing substances in the liquid medium to a gas by aeration, distillation, stripping or other means. Thus, the process of the present invention can be used to treat a liquid medium containing reducing substances. It is normally more advantageous in terms of energy to heat a gas than to heat a liquid to hydrothermal reaction conditions because the specific heat of gas is far lower than the specific heat of water. Therefore, it is sometimes economically more advantageous to once transfer reducing substances contained in a liquid medium into a gas and then treat them by hydrothermal electrolysis rather than to directly treat the liquid medium by hydrothermal electrolysis. This is especially the case with low-molecular weight reducing substances and volatile reducing substances or reducing substances that can be readily transferred from an aqueous solution to a gas phase. However, this is not the case with reducing substances that cannot be readily transferred to a gas phase such as high-molecular weight substances because enormous energy is required to transfer them into a gas phase.
A second aspect of the present invention provides an apparatus for smoothly performing this process for treating a gas by hydrothermal electrolysis.